This invention relates to a method for preventing water vapour in humid air in an inner space of a monocoque structure which has an outer, at least an essentially hermetic structure and an inner partition which is devised to provide a space adjacent the hermetic shell, e.g., a pressure cabin of an aeroplane, from condensating into water and possibly freezing into ice on the inside of the outer hermetic shell which constitutes the monocoque structure and in the space by the shell, and for removing existing water, if any, from this space. Although primarly intended to be applicable to aeroplanes, the invention is applicable to stationary as well as transportable structures, e.g., constructions, containers and ground vehicles.
A pressure cabin of an aeroplane constitutes an essentially hermetic monocoque structure which is pressurized through a flow of heated air from the aeroplane's engine compressors and is directed into the cabin after the regulation of pressure, temperature and humidity. The engines of the aeroplane must therefore be operating, wherefore this type of pressurisation normally is done only during flight. A similar system, which is however preferably meant for being stationed on ground, is described in U.S. Pat. No. 3,623,332. Systems of this kind are intended for creating a conditioning of air which is blown into the aeroplane's pressure cabin in order to provide good comfort to the aeroplane's passengers and crew without considering the possible condensation of the water vapour which exists in the air. These systems have consequently not been utilised in order to eliminate damage to the monocoque structure of the aeroplane or to other vital parts of the aeroplane which are caused by water which condenses in the aeroplane.
Concurrently with the increasing costs of new produced aeroplanes, it has become an aim among the airlines to extend the aeroplane's life expectancy beyond the originally projected life expectancy. Components such as engines and other equipment therefore can be overhauled and exchanged continuously during the use of the aeroplane, whereas worn and damaged parts of the aeroplane body have turned out to be more difficult and time-consuming to repair and also contribute to extra weight. Damage to the aeroplane body is mostly caused by corrosion, mainly on the aeroplane's pressure cabin, which is comprised of a hermetic shell, which on the inside maintains an environment with a high humidity arising from i.a. the water each passenger emits during the stay in the aeroplane, and which on the outside is exposed to very low air temperatures. The outer shell of the pressure cabin is further complemented with a heat-insulated inner partition extended along the mentioned shell. There is no diffusion barrier preventing water vapor between the inner partition and the outer shell, wherefore the humid air in the cabin reaches the cold outer shell unimpededly, where the water in the air condensates and possibly also freezes into ice.
In a known aeroplane accident where the roof of the pressure cabin was torn off in the air, the reason for the insufficient strength was that the pressure cabin had been exposed to very severe corrosion, which had contributed to a decrease in the fatigue strength of the pressure cabin.
In addition to damage to parts of the aeroplane body itself, the condensed water also causes damage to other components and foremost to electrical apparatuses. Even mould and fungus assaults can be found in humid areas of an aeroplane. To prevent the occurrence of such damage, it has been necessary to enclose these components in water-proof covers and similar devices, which has made not only the cost of the structures higher but it has also increased the aeroplane's weight. Because of the pressure changes in the aeroplane, humid air penetrates into the components in spite of these measures and causes damage.
Experience has also shown that an aeroplane of the conventional type for 120-150 passengers contains at least 500 kg water in free form on free surfaces and in cavities and similar places, as well as absorbed in the insulation of the pressure cabin and in hygroscopic materials. Even considerably larger amounts of water can be collected in the aeroplane during flight, e.g., in the form of ice, which when melted must be drained or be removed in another way before the aeroplane can be taken into traffic again. The increase of the aeroplane's futile load in the form of condensed water, which can not be removed during the ground stops, is therefore a considerable negative factor.
In order to solve the condensation problem in aeroplanes, attempts have been made to use ground-based dehumidifying equipment which is connected to the aeroplane when stationed on ground. During the dehumidification on ground, which also is very time-consuming, the cabin must be entirely closed, which implies that the dehumidification is difficult to carry out at the same time with technical work being carried out inside the aeroplane.
Another solution which has been applied to diminish corrosion damage from condensated water is to treat the surfaces being exposed to corrosion with water-proof or water-repelling materials. However, up to now these methods have not lead to any success, instead inspections and repairs must be carried out in a continously increasing frequency the older the aeroplanes become.
A general known procedure to make water in humid air to condense is to let the humid air pass cold surfaces with a temperature below dew point. This procedure is applied in, e.g., known freon-based air conditioning equipment in housing, vehicles, etc.